1. Technical Field of the Invention
The present invention relates to a technology for processing a sound signal provided to a sound emitting device.
2. Description of the Related Art
There have been suggested technologies in which the volume of a sound signal provided to a sound emitting device is automatically controlled according to an observation signal produced from an observation sound that a sound receiving device has received from the sound emitting device. For example, Patent References 1 and 2 have disclosed technologies in which an adaptive filter generates an adaptive signal so as to minimize the magnitude of an error signal corresponding to the difference between the observation signal and the adaptive signal, and the volume of the sound signal is controlled according to the magnitude of the error signal. Ideally, the error signal corresponds to a noise component of the observation sound. Thus, for example, the sound signal is controlled so that the volume of a playback sound from the sound emitting device is raised when the magnitude of the error signal is higher than a reference value for volume control (i.e., when significant noise is present in the environment), and the volume of the playback sound is reduced when the magnitude of the error signal is less than the reference value.    [Patent Reference 1] Japanese Patent Application Publication No. Heisei 1-248799    [Patent Reference 2] Japanese Patent No. 3286981
3. Summary of the Invention
However, in the technologies of Patent References 1 and 2, the magnitude of the error signal may deviate from the magnitude of the actual noise component. For example, when the magnitude of the noise component is significantly lower than the magnitude of a sound that the sound receiving device has received from the sound emitting device, a component other than the noise component in the observation signal may remain in the error signal due to limited capabilities of a subtracter that subtracts the adaptive signal from the observation signal. In such a case, when the magnitude of the error signal is higher than the reference value, the sound signal is controlled to increase the volume of the playback sound and therefore the magnitude of the error signal is further increased. Accordingly, the volume of the playback sound from the sound emitting device continues increasing even though the volume of the playback sound is sufficiently high. In addition, since the magnitude of the error signal including the component other than the noise component is higher than the magnitude of the actual noise component, there is a problem in that the gain of the volume of the sound signal is set somewhat higher than the original level so that the volume of the playback sound is not reduced to an optimal level suitable for the original noise component (or it takes a certain amount of time to reduce the volume of the playback sound to the optimal level).